In devices for processing of a paper web, such as, for example, paper machines and coating machines, cut-off devices are used, whose function is to cut off the web at a number of points when some disturbance of operation takes place in the machine, such as, for example, an uncontrolled web break. The purpose of the cutting off is to protect components that might be damaged when the paper web proceeds as wrinkled and having lost its tension inside the machine, for example, through a coating station or through a calender with soft rolls.
Further, a cut-off device is employed in reelers as a part of the machine reel change equipment. At present, for cutting off a paper or board web, among other things, the following methods are used:
Board and paper grades whose basis weight is high, typically &gt;150 g/m.sup.2 (grams per square meter), are cut off by using a so-called bag change or a cutting band or a cutting string. In bag change, the machine reel that is being completed is slowed down while, at the same time, pressurized air is blown to below the web. The web rises and forms a bag and enters into the nip. A sudden jerk cuts off the web. In change by means of a band, a separate band or string is fed to which an adhesion face has been fitted, for example, by means of an adhesive tape at the inlet side of the nip so that the adhesive tape affixes the band to the face of the reel spool outside the web, and the web tears the web apart in the nip while winding the web in spiral shape around the reel spool. In band change devices, the band that cuts off the board or paper web remains in the interior of the machine reel that is being completed on the face of the reel spool, where it produces bottom broke. The board/paper is glazed at the band over a thickness of several tens of windings. Nor is the reliability of operation of band change devices good, and their operation requires an abundance of manual work and great care.
With medium-weight paper grades, from about 80 to about 150 g/m.sup.2, a so-called swan-neck change is employed, in which a little cut is cut into the web before the nip, and after the nip pressurized air is blown to the cut area, in which connection the web is torn up to the edges.
With thin paper grades, about &lt;100 g/m.sup.2, in reelers of the Optireel.TM. type marketed by the current assignee's, blowing from below the web or from the side is employed in order to pierce and to cut off the web. In particular with thin paper grades and at high running speeds, cutting off of a paper web by blowing produces detrimental paper chips, because the process of cutting is not controlled. With thicker paper grades, when blowing exclusively by means of pressurized air is employed, problems arise because the pressurized air cannot pierce the web or does the piercing untidily.
Further, there are applications in which a high-pressure water jet/jets is/are employed as the cutting element, which jets move across the web at a high speed. A problem of water-jet cutting is the necessity of very quick linear movements in the cross direction of the web and long cutting tails. In water-jet cutting, uncontrolled web breaks may also arise, in which case the web is torn by itself further from the cut that has been made by the jet.
There are also applications in which various mechanically striking blades are employed, which cut off the web as of full width.
The current assignee's FI Patent No. 97,339 constitutes the prior art most closely related to the present invention. In said patent, a cut-off device based on a mechanical blade is described. Said cut-off device includes a cut-off blade and an actuator that is fitted to act upon the cut-off blade to produce a cut-off stroke. The actuator consists of a number of cylinders fitted inside a chamber, each of which cylinders includes a piston and a piston rod. The piston rods have been attached with a certain spacing to a cut-off blade extending across the whole web width. A rapid stroke of the cut-off device is produced by means of rapidly opening pressure-controlled control valves of large area for the cylinders attached to the cut-off blade and by placing the cylinders in the interior of a pressure chamber so that the path of the compressed air from the pressure chamber into the cylinders is as short as possible.
In the cut-off device described in the FI Patent 97,339, variation may occur in the cutting speed. This comes at least from wear of the seals of the retainers and/or from problems related to the two-part structure of the instant-discharge principle used for the discharge.
However, when it operates correctly, cutting off taking place by means of a blade involves a number of advantages, such as controlled cutting off and tidy cutting result also at high speeds, as well as easy regulation of the cutting capacity by means of regulation of pressure in compliance with the paper grade to be cut off.